This invention relates to refractive/diffractive intra-ocular lenses having a plurality of focal lengths.
Intra-ocular lenses ("IOL's") are routinely used as replacements for cataract impaired natural lenses. An ophthalmic surgeon removes the damaged natural lens and implants the artificial IOL. An IOL is simply a plastic lens that has the same optical power as the natural lens that it replaces. The main difference between the natural lens and an IOL is the inability of an IOL to change its focal length. The natural lens is pliable, and its shape is controlled by muscles in the eye. The change in shape of the lens causes a change in the focal length of the lens. The natural eye is therefore able to focus over a range of distances, from approximately 15 inches to infinity. As perceived by the eye an object is at a distance of infinity if it is about 12 feet or further away. The IOL's shape, on the other hand, is fixed, and can't be changed by muscles in the eye. Consequently, the resulting focal length of the IOL is also fixed. An eye that has an IOL is, therefore, no longer able to accommodate its focus. In the majority of cases, the IOL recipient must wear bifocal eyeglasses in order to see clearly at both near and far distances.
Numerous designs of IOL's have been developed in order to provide more than one focal length. The multi-focal designs can be classified into two groups, either completely refractive or refractive/diffractive. In either case, the incident light is split in such a way that a certain percentage of light is focused at different distances. Refractive/diffractive designs have the advantage over refractive designs in that they can be designed to be insensitive to decentration in the eye as well as to the pupil size of the eye.
It should be stressed that the image quality of a multi-focal IOL suffers from a loss of contrast. The fraction of the light that is not in-focus for a particular image distance contributes to a background bias. For example, consider a bifocal IOL that has been designed to produce two foci, with 50% of the incident light focused in each one. At either foci 50% of the light will produce an out-of focus image, that is perceived by the eye as an approximately constant background bias, resulting in an image contrast of about 50%. Ideally, one would like to have an IOL that could produce focused images at many different distances, yet this can result in an increase in image bias as a function of the number of focal points. For example, an IOL with ten focal points, each containing 10% of the incident light, would result in a maximum image contrast of around 10%. Thus there exists a point where the advantages of having a number of foci are negated by the loss of image contrast. Fortunately, depending upon the parameters of the IOL, some of the out-of-focus light can contribute positively to the image. The exact contrast of the image can be determined by quantitative analysis.
Quantitatively, the image contrast is expressed by the modulation transfer function ("MTF") of the IOL. The MTF can be used to determine what the contrast will be for any given image distance and spatial frequency. The MTF by itself is not enough information to determine whether or not a design would be acceptable as an IOL. The visual factors, both physiological and psychological, play a large part in determining the acceptability of an image.
U.S. Pat Nos. 4,210,391; 4,995,714; 4,995,715; 5,121,980; 5,121,979; and 5,129,718, all issued to Cohen ("Cohen Patents"), teach the use of a phase zone plate having a plurality of annular regions that direct light to two foci and rely upon simultaneous vision to discard unfocused images. They disclose the use of alternating concentric Fresnel zones having a generally saw-toothed design to diffract the incident light such that two focal points are produced in the 0.sup.th and 1.sup.st order each of which contains about 40% of the incident light. Thus, at either of the image locations for which the IOL was designed, 40% of the light will produce a focused image, 40% of the light will produce an out-of-focus image, and the remainder of the light goes into other focal points. The out-of focus image will be perceived by the eye as an approximately constant background bias, resulting in an image contrast of about 40%.
U.S. Pat. Nos. 5,076,684; 5,116,111; and 5,129,718, issued to Simpson et al., Simpson et al., and Furhey et al., teach a refractive/diffractive bifocal ophthalmic lens having a phase zone plate embedded within the lens such that the anterior and posterior surfaces of the lens are smooth. The diffractive zone is of a generally saw-toothed or stepped design, and the zones are not of equal area. For instance the central zone is made smaller than the other zones in order to ensure adequate functioning of the lens in a plurality of light conditions.
A "phase zone plate", as used herein, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets to produce a specific wavefront which results in a specific intensity distribution of light at the various order foci of the zone plate.
At present, only one multi-focal IOL has been approved for use by the FDA. The IOL, based on U.S. Pat. No. 4,637,697 issued to Freeman and produced by Minnesota Mining and Manufacturing Company ("3M"), is a refractive/diffractive bifocal design. The diffractive surface profile is made such that it produces two focal points each containing 40% of the incident light. The residual 20% of the light goes into other focal points, and essentially results in an additional image bias. Therefore, each of the two useful foci form images with a maximum contrast of 40%. The important point is that this design, with a 40% maximum contrast, has been tested in vivo, and found to be acceptable. The MTF of the 3M design can be used as a baseline to evaluate the potential acceptability of other designs.